This year marks the 150th anniversary of Cinco de Mayo, and many of us still haven’t learned what it stands for. It’s become one of those vaguely ethnic holidays that we get all bungled, like our annual homage to St. Patrick: a manic wearing-o’-the-green (www.history.com notes that the original color associated with St. Patrick was blue). Not coincidentally, these are both excellent marketing opportunities, with a plethora of food specials and drink specials and deejay dance parties.